The present invention relates to fluid diagnostic techniques and in particular to non-obtrusive diagnostic techniques which enable the measurement of different parameters in a gas flow.
Optical techniques are desirable for the measurement of fluid parameters as other instruments, such as physical type probes, disturb the flow at precisely the location of measurement.
It is known to measure fluid parameters remotely using laser techniques. When a laser of sufficient power is focused tightly in a gas a small volume of the gas around the focus is ionized. This ionization, known as laser induced breakdown is accompanied by the emission of light and sound.
A considerable amount of light from the laser is scattered by the breakdown spark. Light emissions from recombination ions and electrons and from exited molecules and ions generated in the breakdown also persist after the initial laser pulse.
The present invention seeks to provide a non-intrusive diagnostic technique which utilizes the laser induced breakdown phenomenon.
According to a first embodiment of the present invention, a fluid diagnostic technique comprises the steps of focusing a laser in the fluid to generate a laser induced breakdown spark, measuring the characteristics of the initial laser induced breakdown spark and comparing the characteristics of the initial laser induced spark to the characteristics of a delayed image of the spark, formed by light emission due to recombination and excited molecules and ions generated by the laser induced spark, differences between the initial and delayed images being used to diagnose characteristics of the fluid.
In a second embodiment of the present invention, a fluid diagnostic technique for measuring the velocity of a fluid flow comprises the steps of focusing a laser in the fluid to generate an initial laser induced breakdown spark, measuring the center of the initial laser induced breakdown spark and comparing the center of the initial spark to the center of a delayed image of the spark formed by light emission due to recombination and exited molecules and ions generated by the initial laser induced spark, the shift in position of the center of the initial spark and the center of the delayed image of the spark being used to calculate the velocity of the fluid flow.
In a further embodiment of the present invention, a fluid diagnostic technique for measuring the temperature of a fluid comprises the steps of focusing a laser in a fluid to generate an initial laser induced breakdown spark, comparing the initial laser induced breakdown spark to a delayed image of the spark formed by the light emission due to recombination and excited molecules and ions generated by the initial laser breakdown spark, the rate of increase of the light emitting volume around the delayed image of the spark being used to calculate the temperature of the fluid.
Preferably a high power laser such as a Neodymium/YAG laser is used to generate the laser induced breakdown spark. The laser may be focused with a lens and preferably a camera is used to record the images of the laser induced breakdown spark so that the images can be compared.
A collection lens at 90.degree. to the laser beam may be used to collect light emitted by the laser induced breakdown spark. The initial spark and the delayed image of the spark are separated by a time interval of the order of 4.5 .mu.s.